Increasing numbers of younger women continue to receive diagnoses of the most dangerous form of skin cancer even as the rate of new cases has leveled off in younger men, federal health officials reported yesterday.

An analysis of government cancer statistics from 1973 to 2004 found that the rate of new melanoma cases in younger women had jumped 50 percent since 1980 but did not increase for younger men in that period.

"It's worrying," said Mark Purdue, a research fellow at the National Cancer Institute, who led the analysis published in the Journal of Investigative Dermatology. "What we are seeing in young adults right now could foretell a much larger number of melanoma cases in older women."

The new research did not examine the reasons for the trend, but Purdue said it could be the result of such factors as women spending more time outdoors and engaging in indoor tanning. Young women are much more likely than young men to frequent tanning salons, Purdue and others noted.

"One possible explanation is increases among young women of recreational sun exposure or tanning bed use," Purdue said. "Both of these things have been identified as risk factors. It's possible increases in these two behaviors may be responsible."

About 62,000 melanoma cases are diagnosed each year in the United States, and more than 8,400 people die of the disease, according to the American Cancer Society. Previous studies have shown that the rate of new diagnoses has been increasing among adults overall, but it was unclear what was happening with younger adults.

Purdue and his colleagues analyzed cancer statistics for men and women ages 15 to 39 collected through the NCI's Surveillance, Epidemiology and End Results (SEER) Program, a network of regional cancer registries.

For young men, the rate of new melanoma cases rose from 4.7 cases per 100,000 per year in 1973 to 7.7 cases per 100,000 per year in 1980, but it then stopped rising.

"The reason for the leveling off in melanoma rates among young men is not known," Purdue said in an e-mail. "This may reflect reductions over time in the amount of sun exposure experienced by young men (public awareness campaigns regarding sun exposure and melanoma were widely launched in the US in the 1980s). However, we really don't know for sure."

For young women, the rate went from 5.5 cases per 100,000 per year in 1973 to 9.4 in 1980, and it kept rising to 13.9 in 2004.

"These findings are important because they suggest that public education campaigns to educate Americans about the risks of skin cancer from sun tanning do not appear to have resulted in a reduction in melanoma rates among young women," Purdue said.

The increase is unlikely to be simply the result of increased awareness and diagnosis, Purdue said, because the data also suggest the cancers are being found at a later stage.

Although better diagnosis cannot be ruled out, Ahmedin Jemal of the American Cancer Society said the increase was probably real. He noted that young women are more likely than young men to use sunscreens, which ironically lead to more sun exposure.

"They may feel more protected and so stay in the sun for a longer time," he said.

C. William Hanke, president of the American Academy of Dermatology, said the findings should serve as a reminder to young women about the dangers of unprotected outdoor sun exposure and indoor tanning.

"The take-home message is: Unprotected outdoor ultraviolet exposure is dangerous," Hanke said. "Ultraviolet radiation is a carcinogen. If you bathe your skin in the ultraviolet light carcinogen long enough, skin cancer is going to develop."

The tanning salon industry disputes assertions that indoor tanning is playing a role, saying the increase in diagnoses may be the result of better screening. The industry also challenges assertions that indoor tanning increases the risk of melanoma.

"For people to talk about indoor tanning as a cause of melanoma shows they haven't looked at the science on the subject," said Sarah Longwell, a spokeswoman for the Indoor Tanning Association. "It's shocking to make such a claim. There has been no scientific studies that show that indoor tanning causes melanoma. It's almost a reckless claim. It's an overt effort to slander the indoor tanning industry."